T cell activation involves the integration of several signals. In addition to antigen specific signaling through the T cell receptor (TCR) and costimulatory signaling through molecules such as CD28, signaling through additional molecules including the transmembrane receptor Notch are critical at many stages of T cell activation and development. To identify novel molecules that regulate T cell activation, we used a genetic approach to rescue the T cell activation defect in a Jurkat mutant cell line by retroviral expression of a leukocyte library. In our initial screen, we found that retroviral expression of NKAP complemented the defect in one Jurkat mutant cell line and restored T cell activation. We have demonstrated that NKAP is a novel negative regulator of Notch signaling that associates with CIR, which is part of the Notch co-repressor complex. To determine the function of NKAP in vivo, we generated mice with a floxed NKAP allele. Lck-cre NKAP conditional knockout mice have a severe block in ??T cell development at the DN3/2-selection checkpoint, although ??T cell development proceeds normally. Interestingly, mice deficient in either NcoR or mSin3a, two generic components of transcriptional corepressor complexes, also have a DN3 block during T cell development, indicating that epigenetic changes are required as cells pass the ?-selection checkpoint to become DP T cells. As predicted, examination of three Notch target genes, CD25, Deltex1 and Hes1 by QPCR demonstrated that NKAP lck-cre cKO DP T cells had increases in gene expression by 8- to 20-fold, proving that the NKAP functions as a negative regulator of Notch signaling in vivo, and is required for T cell development. Analysis of T cell development in CD4-cre NKAP cKO mice demonstrates a block within SP thymocytes as they progress from semimature to mature. In addition, there are decreased numbers of T cells in the periphery in CD4-cre NKAP cKO mice, and these naive T cells express markers consistent with being recent thymic emigrants, indicating that NKAP also plays a critical role in T cell maturation. This proposal will focus on understanding the function of NKAP during T cell development and maturation, to uncover how altered gene regulation in the absence of NKAP alters T cell development and to define the mechanism whereby loss of NKAP leads to upregulation of Notch target genes. Specific Aim #1: Regulation of T cell development and maturation by NKAP Specific Aim #2: Mechanism of altered gene expression in the absence of NKAP